The present invention relates to an objective optical system and an endoscope. More particularly, the present invention relates to an endoscope to be combined with a laser probe for use.
In endoscope systems, an endoscope such as a ureteroscope is combined with a laser probe for use. A ureteroscope is employed to crush a renal pelvic calculus by lithotripsy from the ureter. A laser probe is driven from the treatment device insertion hole of an endoscope and a laser beam is irradiated to lithotripsy. The position of the front end of the probe is indicated by red guiding light but the halation produced by such guiding light gives rise to a problem to be solved. Additionally, the objective optical system needs to be downsized as much as possible in order to require the fine outer diameter of about ø3 mm. Furthermore, all the components including the CCD unit need to be downsized from the viewpoint of video-recording.
Small objective optical systems formed by using two positive lenses are known. On the other hand, an infrared cutoff filter has to be used to reduce the halation produced by guiding light. Thus, the lens itself is formed by an absorption type infrared cutoff filter in order to maintain the small size in some of such small objective optical systems.
However, when the lens itself is formed by an absorption type infrared cutoff filter, such a filter is not suited for mass production because the processability of the filter is poor from the viewpoint of hardness and easy abrasion. Additionally, when a lens formed by using an absorption type infrared cutoff filter is employed as first lens, it gives rise to problems in terms of durability and adaptability to living bodies. On the other hand, when a lens formed by using an interference type infrared cutoff filter is employed, its characteristics change as a function of oblique incidence so that a space needs to be provided in order to make parallel light enter the filter. Then, such a space by turn baffles the efforts for downsizing the objective optical system.
An optical system formed by arranging a positive first group and a positive second group sequentially from the side of the object, of which the first group is formed by bonding three components including a parallel flat plate, an infrared absorption filter and a plano-convex lens that are arranged in the above mentioned order from the side of the object, an aperture diaphragm being arranged between the infrared absorption filter and the plano-convex lens, has been disclosed (Patent Document 1).
Patent Document 1: International Patent Publication No. WO99/06866
However, in the optical system disclosed in the Patent Document 1, the diaphragm needs to be arranged at the image side of the infrared absorption filter or on the flat surface of the plano-convex lens by vapor deposition, which is a poor processability operation, and is not suited for mass production. Additionally, the Patent Document 1 does not describe anything about conditions for obtaining a good image quality.
In view of the above-identified problems, it is therefore the object of the present invention to provide a small objective optical system showing good color reproducibility and a reduced level of the halation due to guiding light of a laser probe.